Add timestamps to API pipelines responses

What does this MR do?

Adds the created_at and updated_at attributes to responses from the API pipelines endpoints.

This is motivated by a specific customer request, see #33099 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content